Method of manufacturing electric conductor insulated by foamed crystalline polymer



United States Patent 3,522,088 METHOD OF MANUFACTURING ELECTRIC CON-DUCTOR INSULATED BY FOAMED CRYSTAL- LINE POLYMER Chisato Kawazoe,Tokyo-to, Terumichi Ichiba, Kamakura-shi, and Seichi Iwaknra and HiroshiShimba, Yokohama-shi, Japan, assignors to Sumitomo Electric Industries,Ltd., Osaka, Japan, a company of Japan Filed June 29, 1966, Ser. No.561,564 Claims priority, application Japan, July 7, 1965, 40/ 40,727Int. Cl. B44c 1/36, 1/42; C08f 29/04 US. Cl. 117-232 Claim ABSTRACT OFTHE DISCLOSURE A method of manufacturing an electric conductor with acoating of a foamed polymer wherein the conductor is coated with asolvent solution of either polythene or polypropylene and dried to causeparticle aggregation 'of the polymer which is physically noted by theclouding of the coating and to remove the solvent remaining. The driedcoated conductor is then immersed in a liquid of monoatomic orpolyatomic alcohol and heated to a temperature which is at least equalto the melting point of the selected polymer to cause foaming. Thefoamed coating is then rinsed in water to remove the residual alcoholand cool the coating.

The present invention relates to an improvement in the manufacture offoamed plastic insulated electric conductors of the type which have ahigh-temperature viscous solution of crystalline polymer, e.g. polytheneor polypropylene, applied thereover which is thereafter treated toprovide closed cells in the interior of coating.

Referring to the prior art illustration of FIG. 1, the conventionalmethod employed for the manufacture of foamed plastic insulated electricconductors is conducted by first coating a viscous solution ofcrystalline polymer, e.g. polyethylene or polypropylene, over theconductor W in a solution coating tank 1. Then the coated conductorpasses through a drying duct 3, which has its interior exposed to theatmosphere, for drying, followed by passing through a foaming furnace(or heating furnace) 4 to produce closed cells in the interior of thecoating, and the foamed plastic insulated electric conductor thusobtained is taken up on a reel 5.

In the above-mentioned method, an electric furnace has been used for thefoaming furnace 4 in which air or other incombustible gases are heatedby electricity. When the conductor coated with a viscous solution ofcrystalline polymer passes through such an electric furnace as describedabove, the coated conductor is heated mainly by high temperature air orother incombustible gases and is partly heated by the heat radiationfrom the wall of the electric furnace. Thus, closed cells are obtainedin the interior of the coating of coated conductor.

However, the heat capacity of air (or other incombustible gases) issmall and furthermore the thermal conductivity of air (or otherincombustible gases) is low, so that the high heating temperatures andlong durations of heating are required for foaming. For instance, ittakes approximately 4 seconds to obtain a coating with an expansiondegree of 2030% by passing the coated conductor through an electricfurnace in which air is heated to 360 C.

Recently, it has been required to increase the line speed of coatingmachines for the purpose of increasing output. In order to increase theline speed by using the conventional foaming furnace, it is necessary toincrease the length of the foaming furnace. As a result of this, thedistance between supporting points of the coated conductor passingthrough the foaming furnace becomes longer, so that the coated conductorvibrates excessively at higher line speeds and consequently can causedamage to the coating. Furthermore, it is difiicult to uniformlymaintain the temperature distribution of gas in the cross-sectional andlongitudinal directions of the furnace. In consequence, it is not easyto obtain a uniform foaming of the coating in its radial direction. Whena number of coated conductors are placed in the furnace at the sametime, it is furthermore not easy to obtain uniformity of the expansionstate among the foamed conductors.

The object of the present invention is to provide a method ofmanufacturing foamed plastic insulated electric conductors, which makesit possible to shorten the time required to foam the interior of thecoating and to obtain a uniformly foamed coating. The present inventionis characterized in that after the viscous solution of crystallinepolymer is coated over a conductor and the coated conductor thusobtained is dried, the coated conductor is dipped in a heated liquidwhich hardly swells and dissolves said coating in order to foam theinterior thereof (referred to as the melting point of the polymer). Itis then rinsed and cooled with hot water or cold water, or both.

FIG. 2 is a block diagram showing the features of the manufacturingmethod of the present invention. Identical numerals and letters denotesimilar items.

After a viscous solution of crystalline polymer is coated over theconductor W in the solution coating tank 1 and the coated conductor thusobtained is dried in the drying duct 3 as described in conjunction WithFIG. 1. The coated conductor is dipped in the heated liquid trough 6 forfoaming and is then rinsed and cooled in the water trough 7, followed bytake-up on a reel 5.

For the heated liquid employed for foaming, any liquid which hardlyswells the coating of crystalline polymer can be used. As a result ofmany experiments performed repeatedly, however, monoatomic or polyatomicalcohols are preferable. Alcohols which may be used as the heatingliquid, one may use ethylene glycol, diethylene glycol, tri-ethyleneglycol, propylene glycol, butylene glycol, pentanediol, hexanediol,glycerin, etc. These alcohols scarcely swell and dissolve the coating ofcrystalline polymer, and can be readily rinsed out with Water.

As for the temperature of the heating liquid, any temperature which isnot lower than the melting point of the crystalline polymer employed inthe coating and which is lower than the boiling point of the heatingliquid is acceptable. For example, the range of preferable tempera atureof] the heating liquid is C. to 250 C. when a viscous solution of highdensity polyethylene is used as the coating solution and glycerin isadopted as the heating liquid.

As one example of this invention, a 28 wt. percent solution of highdensity polythene in xylene, which was kept at approximately C., Wascoated to 0.55 mm. in over-all diameter over a 0.32 mm. copper conductorand then the coating was dried. Foaming was then performed on threedifferent lengths of coated conductor by immersion in a glycerin bath ata temperature of 180 C. for 0.2 second, 0.5 second and 1 secondrespectively. The expansion degree of the coatings of these threeelectric conductor insulated by foamed polyethylene was measured. As aresult of the measurements, it has been ascertained that the expansiondegree of 20-30% having been obtained with each of these three kinds wasno different from those that were foamed by heating in an electricfoaming f urnace for about 4 seconds by use of the conventional method.

The above-mentioned manufacturing method by the present invention hasthe following advantages:

(1) Compared with the method hitherto employed in which a heatingfurnace, such as an electric furnace, is used and a coated conductor isheated by the heated air (or other incombustible gas) in the interior ofthe furnace and by heat radiation from the wall of the furnace, themethod of the present invention in which a heating liquid is used,enjoys a greater heat capacity and heat transfer effect, so that it ispossible to remarkably shorten the heating. It also makes it possible toemploy a lower heating temperature for foaming.

(2) As the duration of heating is short, it is possible to shorten thelength of the heating trough for foaming. This results in reduction ofthe distance between supporting points of the coated conductor. Inaddition, there is almost no chance in causing damage to the coatingbecause the' vibration of coated conductor in a liquid is much less thanin a gas due to the damping effect of liquid.

(3) As the heating liquid used for foaming is compatible with water, itis easy to remove the residual liquid,

which remains on the surface of the insulated conductor after foaming,with water.

(4) As the heat capacity and thermal conductivity of liquids are greaterthan those of gas, it is easy to obtain a much more uniform temperaturedistribution and temperature control throughout the heated liquidtrough. This results in obtaining a more uniformly foamed coating ofcrystalline polymer.

What we claim-is:

1. The method of manufacturing an electric conductor with a coating of afoamed polymer comprising the steps of coating the conductor with asolvent solution of a crystalline polymer selected from the groupconsisting of polythene and polypropylene, drying the coated conductor,immersing the dried coated conductor for not more than one second in aliquid selected from at least one in the group consisting of monoatomicalcohol and polyatomic alcohol and heated to a temperature of at leastequal to the melting point of the selected polymer to cause foaming inthe coating, and thereafter rinsing the foamed coated conductor withwater to remove the residual liquid and cool the foamed coating.

References Cited UNITED STATES PATENTS WILLIAM D. MARTIN, PrimaryExaminer D R. M. SPEER, Assistant Examiner U.S. Cl. X.R. 1l7-119.8

